Scheduled health check for a server

healthCheck.js

/*
The idea is that the machine is used by a single cloud function / endpoint, which is called either periodically or based on 

- Health check using periodic process, such as Google Cloud Scheduler or cron
- On failure, schedule additional checks, for ex. using Google Cloud Tasks
- If failures continue, restart the system
- Go to normal mode of operation if all good
*/

// To visualize statechart, copy paste code to https://statecharts.github.io/xstate-viz/
// Available variables:
// Machine (machine factory function)
// assign (action)
// XState (all XState exports)

const healthCheck = Machine({
  id: 'healthCheck',
  context: { attempts: 0 },
  initial: 'normal',
  states: {
    normal: {
      onEntry: 'logOK',
      on: {
        ERROR: 'escalated',
        OK: 'normal',
      }
    },
    escalated: {
      onEntry: [
        assign({ attempts: 0 }),
        'scheduleRecheck',
        'logError',
      ],
      on: {
        OK: 'normal',
        ERROR: [{
          cond: 'maxAttempts',
          target: 'restarting',
        }, {
          actions: [
            assign({
              attempts: ctx => ctx.attempts + 1
            }),
            'scheduleRecheck'
          ],
        }]
      },
    },
    restarting: {
      onEntry: ['restart', 'logRestart'],
      on: {
        OK: 'normal',
      },
    },
  }
}, {
  guards: {
    maxAttempts: ctx =>  ctx.attempts >= 2
  }
});